FIG. 1 is a schematic diagram illustrating a conventional AC/DC power converter. In FIG. 1, components within dashed box 101 are integrated on a silicon die. An AC voltage is applied to input ports 102 and 104, and a regulated DC output voltage with respect to ground 108 is provided at output port 106. A transformer that includes primary winding 110 and secondary winding 112 is illustrated in FIG. 1, and the transformer also comprises another secondary winding 114 for charging capacitor 116 during steady state operation.
Various components within silicon die 101 require a rectified voltage for operation. During steady state operation, the voltage across capacitor 116 provides at node 122 the voltage needed by pulse width modulator (PWM) 118. During steady state operation, PWM 118 switches switching Metal Oxide Semiconductor Field Effect Transistor (nMOSFET) 120 at a rate sufficient to energize primary winding 110 so that a feedback voltage is adjusted to equal a reference voltage (not shown).
During start up, capacitor 116 has not yet been charged up, so that the voltage developed at node 122 is initially low, e.g., proximately equal to ground. To develop the voltage needed during startup, a high voltage internal current source is utilized. Node 122 is connected to the gate of nMOSFET 124 and to the source of nMOSFET 126. The source of nMOSFET 104 is biased by voltage source 128. With the voltage at node 122 low, nMOSFET 124 is off and nMOSFET 126 is on. With the drain of nMOSFET 126 connected to node 130, the drain of nMOSFET 126 is at a relatively high rectified voltage, so that nMOSFET 126 serves as a high voltage current source when on, sourcing current to capacitor 116 during startup.
When the voltage at node 122 is high enough to turn on nMOSFET 124, the gate of nMOSFET 126 is pulled low so that nMOSFET 126 is turned off. At this point, PWM 118 starts to operate so that the AC/DC converter enters into a steady state operating mode in which nMOSFET 120 is switched on and off to regulate the output voltage. During steady state operation, secondary winding 114 and diode 132 maintain capacitor 116 charged.
As clearly shown in FIG. 1, the circuit includes two high-voltage pins 134 and 136, connected to the drain of nMOSFET 126 and to the drain of nMOSFET 120, respectively. Manufacturing the individual high-voltage pins 134 and 136 on the silicon die can be expensive and complex. Accordingly, there is a need for a circuit with reduced complexity and cost of manufacturing.